The pinocytosis of the lysosomal enzyme, alpha-L-iduronidase, into cultured fibroblasts is a receptor-mediated process apparently requiring a phosphorylated sugar on the enzyme. The biosynthesis of that marker in normal cells from patients with Mucolipidoses II and III will be examined. The role of secretion and pinocytosis in the transport of enzymes to lysosomes will be investigated. A micro-assay for iduronate sulfatase in hair follicles may provide the basis for detecting heterozygotes of the Hunter syndrome. Purification of that enzyme from plasma has been undertaken in the hope of preparing therapeutically useful concentrates.